Use of valve assemblies and flow control valves for controlling fluid flow from, e.g., power-assist pumps, is well known. The valve assembly provides for uniform flow of hydraulic fluid from the power-assist pump to, for example, an actuator or other fluid operated device.
Generally, a flow control valve comprises a housing having a bore therein forming a flow path between an inlet and an outlet port, with a slidably disposed sleeve or other member arranged to occlude the outlet port when fluid flow is to be prevented. This slidable member or sleeve is, conventionally, biased by means of a spring or the like.
In various known arrangements, the spring is arranged to ensure that, in the normal state, the flow path to the outlet port is open such that a fluid can flow to the inlet port, through the bore and out of the outlet port to control the actuator or the like. If, however, the fluid flow rate exceeds a desired rate, a pressure differential is created in the valve assembly to cause the sliding member to move against the force of the spring so as to occlude the outlet port or to otherwise cause relative movement in relation to the bore so that there is a non-alignment in the fluid flow path from the inlet through the bore to the outlet, thus preventing the flow of fluid through the valve assembly.
There are many applications of hydraulic power where there is a need to regulate the maximum rate of fluid flow. In aircraft applications, for example, the industry has specific safety regulations concerning the maximum flow rate.
Whilst, as mentioned above, hydraulic valves find many applications in a wide range of industries, the control valve in the present disclosure will be described, by way of example and not limitation, in relation to the use of hydraulic actuation systems in aircraft.
Many moving parts in aircraft are controlled using hydraulics. In view of the very high safety requirements in aircraft, such hydraulic systems include a maximum flow requirement. The flow limiter of the present disclosure can be used in any hydraulic system as a flow control valve, wherever there is a requirement for limiting a flow rate.
The specific examples described below relate to the use of a flow control valve in a thrust reverser hydraulic system but this is purely by way of example.
FIGS. 1A and 1B, described again below, show, in cross-section, a conventional flow limiter valve such as might be used in a thrust reverser actuation system.
The flow limiter valve shown in FIGS. 1A and 1B comprises a housing 4 containing a spool 5 arranged on a guide 6 such as to be axially moveable within the housing.
Fluid flows through the flow limiter valve by entering the valve at the input orifice 1, passing through a bore in the sliding spool and exiting at output orifice 3 in the circumferential wall of the housing, via an outlet 2 in the spool.
The sliding spool 5 is biased, by means of a spring 7 such that, under normal flow conditions, the spool outlet 2 is aligned with the output orifice 3 in the wall of the housing. Thus, a flow conduit or path for the fluid is provided from input orifice 1, via the bore and outlet 2 through output orifice 3.
Should the fluid flow exceed a given flow rate or pressure, this creates a pressure differential across the input orifice such as to overcome the spring bias, causing compression of the spring such that the sliding spool moves axially moving the spool outlet out of alignment with the outlet orifice 3. This thus closes off the flow path through the valve. FIG. 1A shows the spool outlet in alignment with the outlet orifice creating a flow path. FIG. 1B shows the slide spool orifice moved out of alignment with the outlet orifice thus closing the fluid flow path.
As can be seen by comparing FIGS. 1A and 1B, to stop flow through the valve assembly, the sliding spool needs to move a distance X (FIG. 1B). This fairly large distance of movement against the force of the spring means that the flow limiting valve responds slower than is desirable to an excessive pressure difference and flow rate. The mass of the sliding spool also needs to be fairly large to move the distance X. This mass results in some lack of stability causing transient spikes in flow rate.
Also, a shorter spool distance results in lower viscous friction force as well as dry friction force.
The aim of the present disclosure is to provide a flow limiter valve that responds more quickly to the flow rate and, hence, pressure differential, exceeding a given value, and in a more stable manner.